He could never explain it, even if he wanted to—no amount of infinite chakra from any Tailed Beast could add to his weak vocabulary—but for a moment there, in between the fight, he lived a future that had not yet happened.
What he saw was this: the fight continued with high-speed sparring. Blows were exchanged almost equally between him and the stranger. A minute or two into the clash, while they were distracted, a group of three genin and their instructor managed to nab the wild boar, which was too petrified to put up any resistance.
"Ah, from your face, I reckon", the stranger began, "you saw it, didn't you? The future. This is why I cannot stay here for long. Things start interceding in ways that cannot be imagined".
"What was that!?", asked Naruto.
"A very advanced form of Observation Haki", he paused, "actually, the less you know, the better".
"What I saw was the future, right?".
"Right", replied the stranger, "those three looked really happy. I think I heard one of them say it was their first successful mission".
"Yeah, it's kind of a big deal for them", said Naruto, "so they're basically hiding nearby, and they come to grab the wild boar when we get distracted fighting".
"Yes", said the stranger, confirming Naruto's understanding, "however, since you saw into that future and withdrew from fighting me, a different future now awaits the wild boar".
"And what would that be?", Naruto said as he narrowed his eyes. He hadn't a clue how to take on this enemy, but he was starting to get real excited.
"Unfortunately, I cannot stay here for long. The fact that you can see into the future already is a telling sign. I have to leave, and before that, I must attend to the business I came here for", said the stranger as he sheathed his sword, "say, those kids looked real happy catching that boar—what say we grown-ups let them have what they want?".
Kurama pressed skip on the Special Ichiraku Ramen slide that was being showcased on the main screen inside Naruto's brain. Instead, he played an image of the time Team 7—Naruto, Sasuke, Sakura, and Kakashi—completed their first mission and how happy they had been for it.
"Fine, let the kids have it", said Naruto.
"Let us leave to a private area then", said the stranger, walking away, "as soon as we leave, the kids will come for the boar, and the boar is too petrified to move—I've made sure of that".
Naruto nodded. For some reason, he found the stranger trustworthy. He also caught another slight glimpse into the future, and the stranger seemed to be telling the truth.
"So what is this business that brought you here?", asked Naruto.
"I came looking for you, Naruto", said the stranger, "did Jiraiya entrust you with anything before he left?".